XelNaga had the right idea. I called Account Support and got everything properly unwound...
a) Only one instance of SC2 will be 'active' on an account. If you add the Retail key first, then later add the CE key, it will "upgrade" your account to CE - but won't free up the retail key. [note that I didn't do this, I read about it in a separate forum thread]
b) I called Account Services when they opened this morning @ 10am. Dozen of people must have gotten in in the seconds before me, because my estimated wait time was 40 minutes... and 80 minutes later (!!) a very helpful human came on.
c) After explaining my situation, they said I could send in the retail materials (DVD, all packaging, + sales receipt) for a refund; they would deactivate the retail key from my account; and then I could add the CE key free and clear.
*** Note: By deactivating the retail key, they erase your SC2 character, including the name you chose and any advancement, ranking, achievements, etc. Lucky for me, I only had 30 profile points. [I suppose I could actually have changed my in-game name when I re-activated, but I'm happy with it as-is...]
d) Instead of getting a refund, I chose to 'gift' my retail key to a friend who had not yet acquired SC2. They asked me for his bnet e-mail, then after deactivating my retail key they added a new key to his account.
In the end, waiting on hold was a PITA but it cleaned up fairly nicely. I'm not too annoyed at having to re-do any progress I had made under the retail key, I've got the extra doodads which came with the CE, and I got to suck a friend into SC2 in the process. ;-)